1. Field of the Invention.
The present invention relates to an apparatus and method for the measurement of fluid flow through a conduit, and more particularly, concerns an apparatus and method for the measurement of air flow through a conduit which accounts for measurement errors due to changes in ambient temperature and caused by velocity cooling of the measurement elements within the conduit.
2. Description of the Prior Art.
In many instances where fluids flow through a conduit, tubing or the like, it is desirable to know the velocity of the moving fluid. For purposes of the ensuing discussion, as well as the scope of the present invention, the term "fluid flow" includes liquid flow as well as gas flow, particularly airflow. In conduit units useful for heating or air conditioning purposes, the velocity of airflow through the ductwork is an important parameter in determining the efficiency and proper operability of the system. Accordingly, a reliable technique for measuring airflow through these heating, air conditioning or like airflow systems is desirable to obtain this important characteristic. Moreover, the measurement of the airflow through the conduit should be accurate, as well as reliable.
Early techniques for determining airflow through a conduit relied upon an instrument known as an anemometer. The anemometer relied upon a windmill effect with a series of cups extending from a rotatable axle. Air captured by the cups would cause the cups to spin thereby rotating the axle, the speed of which could be measured to correspond to velocity of the air moving through the system. For heating, air conditioning or the like ductwork systems, however, a mechanically operated instrument such as an anemometer would not always be feasible or practicable.
Early electronic anemometers deviated from the mechanical spinning technique and relied upon electrical resistance for measuring airflow. For example, an electrically conductive wire would be positioned within the conduit and exposed to the air flowing therethrough. Passing an electrical current through this wire would cause the wire to increase its temperature. Since the airflow through the conduit causes a cooling effect once air passes the wire, a drop in temperature of the wire would result. Concurrent with this drop in temperature of the wire would be a change in electrical resistance of the wire. This change in electrical resistance could be related to the velocity of air passing the wire. Such an early anemometer had a number of deficiencies, including only point measurement within the cross-sectional plane of the conduit, and no compensatory factors which took into account the temperature of ambient air or the like.
An improved electronic anemometer was described by Pollack in the Feb. 1957 edition of Popular Electronics. Instead of an electrically heated wire element, the improved electronic anemometer relied upon a modified Wheatstone bridge in which two thermistors formed a part of the bridge and were closely matched in nominal resistance so that the bridge could be balanced. Current flowing through the thermistors caused them to heat; one of the thermistors was exposed to moving air, which caused the thermistor to cool and its resistance to rise sharply. Accordingly, the Wheatstone bridge balance was changed, causing the meter to indicate a reading. The amount of the meter deflection was determined by the magnitude of the resistance change, which, in turn, was a function of airflow velocity.
While these electronic anemometers are able to measure airflow, accuracy, nevertheless, remained a problem. Some of these problems were addressed in U.S. Pat. No. 4,494,406, wherein one of the inventors of that patent is the inventor of the present invention. The problems addressed in the patented invention included the use of a plurality of measuring resistors, connected in electrical series, and positioned at various locations in a given plane across the fluid flow conduit. Such a resistor arrangement purportedly improves accuracy since many locations of the flowing air are sampled to obtain the velocity determination. Another area addressed by the patented invention is in the use of a reference resistor sensitive only to changes in ambient temperature. The resistance value of the reference resistor is subtracted from the resistance of the measuring resistors to produce a resistive value in terms of fluid flow.
While these improvements in the measurement of airflow through a conduit are noteworthy, problems still persist. Most significantly, the prior art has failed to recognize that the measurement of airflow is a function of the rate of cooling of the heated resistor element by the moving air. Further, the ambient temperature affects the cooling rate of the resistor element as well as changing its resistance due to its temperature versus resistance characteristics. The aforementioned Wheatstone bridge circuit compensates only for the latter phenomenon, i.e., airflow at constant temperature. In this regard, the effects of increased rates of heat transfer related to the combined effects of temperature and airflow (velocity) are not accounted for in the prior art devices. As a result, errors in measurement occur with changes in ambient temperature.
Other airflow measurement apparatuses are described in U.S. Pat. Nos. 4,214,478; 4,196,622; and 3,951,113. These patented airflow measuring apparatuses also fail to account or compensate for the combined effects of temperature and airflow cooling on the measuring elements.
It is, therefore, the purpose of the present invention to provide an apparatus and method for the measurement of airflow through a conduit which accounts for and compensates for the combined effects of temperature and airflow cooling of the measuring element.